Several million people sheltered and prayed for the best on Wednesday as one of the fiercest cyclones in decades roared towards Bangladesh and eastern India, with forecasts of a potentially devastating and deadly storm surge.

Authorities have scrambled to evacuate low lying areas in Amphan's projected trail of destruction, only the second "super cyclone" to form over the Bay of Bengal since records began.

But their task is complicated by the need to follow precautions to prevent the spread of the coronavirus, with infection numbers still soaring in both countries and hospitals struggling to cope.

"At least 50 people took shelter in my concrete-built house," said Abdur Rahim, a Bangladeshi shrimp farmer on the edge of the Sundarbans mangrove forest in the projected path of the storm.

"There is panic. The women are worried," he told AFP. "A few months ago Cyclone Bulbul smashed our village, destroying at least 100 homes. We hope Allah will save us this time."

The vast vortex is visible from space and was 125 kilometres offshore and forecast to slam into the coast in the afternoon or evening.

Gusts of 185 hm/h were predicted when it does, the Indian Meteorological Department said, the equivalent of a Category Three hurricane, with a storm surge of several metres.

Bangladeshi forecasters said it would hit around 18:00, predicting a surge of seawater up to five metres high.

Water

Such walls of water can cascade several kilometres inland, and are often the main killers in any cyclone, typhoon or hurricane.

The Indian met office warned of possible flying objects, "extensive" damage to communications and power lines, and trees being ripped out of the ground by the wind.

Kolkata was battered by heavy rain and the muddy Hooghly river was rising under dark skies, while in the coastal resort of Digha, large waves were pounding the shore.